Arthur Jeffrey Dempster (August 14 1886 - March 11 1950) was a
CanadianCanada is a country occupying most of northern North America, extending from the Atlantic Ocean in the east to the Pacific Ocean in the west and northward into the Arctic Ocean...
-
AmericanThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
physicistA physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...
best known for his work in
mass spectrometryMass spectrometry is an analytical technique for the determination of the elemental composition of a sample or molecule. It is also used for elucidating the chemical structures of molecules, such as peptides and other chemical compounds...
and his discovery of the
uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. Besides its 92 protons, a uranium nucleus can have between 141 and 146 neutrons. The most common uranium isotopes are U-238 and U-235 . A uranium atom has...
isotopeIsotopes are different types of atoms of the same chemical element, each having a different number of neutrons. Correspondingly, isotopes differ in mass number but not in atomic number. The difference in the number of nucleons comes from a difference how many neutrons are in the atomic nucleus...
235UUranium-235 is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238 it is fissile, i.e. it can sustain fission chain reaction...
.
Dempster was born in
TorontoToronto is the most populous city in Canada and the provincial capital of Ontario. It is located in Southern Ontario on the northwestern shore of Lake Ontario. With over 2.5 million residents, it is the fifth most populous municipality in North America...
,
CanadaCanada is a country occupying most of northern North America, extending from the Atlantic Ocean in the east to the Pacific Ocean in the west and northward into the Arctic Ocean...
. He received his
bachelor'sA bachelor's degree is usually an academic degree awarded for an undergraduate course or major that generally lasts for four years, but can range from two to six years depending on the region of the world...
and
master's degreeA master's degree is an academic degree granted to individuals who have undergone study demonstrating a mastery or high-order overview of a specific field of study or area of professional practice...
s at the
University of TorontoThe University of Toronto is a public research university in Toronto, Ontario, Canada, situated north of the city's Financial District on the grounds that surround Queen's Park. The university was founded by royal charter in 1827 as King's College, the first institution of higher learning in the...
in 1909 and 1910, respectively. He travelled to study in
GermanyGermany , officially the Federal Republic of Germany , is a country in Central Europe. It is bordered to the north by the North Sea, Denmark, and the Baltic Sea; to the east by Poland and the Czech Republic; to the south by Austria and Switzerland; and to the west by France, Luxembourg, Belgium,...
, and then left at the outset of
World War IWorld War I , also known as the First World War, the Great War, and the War to End All Wars, was a global military conflict which involved most of the world's great powers, assembled in two opposing alliances: the Triple Entente and the Triple Alliance...
for the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
; there he received his
Ph.D.Doctor of Philosophy, abbreviated PhD , for the Latin , meaning "teacher of philosophy", or alternatively, DPhil, for the equivalent , is an advanced academic degree awarded by universities...
in physics at the
University of ChicagoThe University of Chicago is a private, coeducational research university in Chicago, Illinois, USA. It was founded by oil magnate and benefactor John D...
.
Arthur Jeffrey Dempster (August 14 1886 - March 11 1950) was a
CanadianCanada is a country occupying most of northern North America, extending from the Atlantic Ocean in the east to the Pacific Ocean in the west and northward into the Arctic Ocean...
-
AmericanThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
physicistA physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...
best known for his work in
mass spectrometryMass spectrometry is an analytical technique for the determination of the elemental composition of a sample or molecule. It is also used for elucidating the chemical structures of molecules, such as peptides and other chemical compounds...
and his discovery of the
uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. Besides its 92 protons, a uranium nucleus can have between 141 and 146 neutrons. The most common uranium isotopes are U-238 and U-235 . A uranium atom has...
isotopeIsotopes are different types of atoms of the same chemical element, each having a different number of neutrons. Correspondingly, isotopes differ in mass number but not in atomic number. The difference in the number of nucleons comes from a difference how many neutrons are in the atomic nucleus...
235UUranium-235 is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238 it is fissile, i.e. it can sustain fission chain reaction...
.
Biography
Dempster was born in
TorontoToronto is the most populous city in Canada and the provincial capital of Ontario. It is located in Southern Ontario on the northwestern shore of Lake Ontario. With over 2.5 million residents, it is the fifth most populous municipality in North America...
,
CanadaCanada is a country occupying most of northern North America, extending from the Atlantic Ocean in the east to the Pacific Ocean in the west and northward into the Arctic Ocean...
. He received his
bachelor'sA bachelor's degree is usually an academic degree awarded for an undergraduate course or major that generally lasts for four years, but can range from two to six years depending on the region of the world...
and
master's degreeA master's degree is an academic degree granted to individuals who have undergone study demonstrating a mastery or high-order overview of a specific field of study or area of professional practice...
s at the
University of TorontoThe University of Toronto is a public research university in Toronto, Ontario, Canada, situated north of the city's Financial District on the grounds that surround Queen's Park. The university was founded by royal charter in 1827 as King's College, the first institution of higher learning in the...
in 1909 and 1910, respectively. He travelled to study in
GermanyGermany , officially the Federal Republic of Germany , is a country in Central Europe. It is bordered to the north by the North Sea, Denmark, and the Baltic Sea; to the east by Poland and the Czech Republic; to the south by Austria and Switzerland; and to the west by France, Luxembourg, Belgium,...
, and then left at the outset of
World War IWorld War I , also known as the First World War, the Great War, and the War to End All Wars, was a global military conflict which involved most of the world's great powers, assembled in two opposing alliances: the Triple Entente and the Triple Alliance...
for the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
; there he received his
Ph.D.Doctor of Philosophy, abbreviated PhD , for the Latin , meaning "teacher of philosophy", or alternatively, DPhil, for the equivalent , is an advanced academic degree awarded by universities...
in physics at the
University of ChicagoThe University of Chicago is a private, coeducational research university in Chicago, Illinois, USA. It was founded by oil magnate and benefactor John D...
. Dempster then joined the
physicsPhysics is a natural science; it is the study of matter and its motion through spacetime and all that derives from these, such as energy and force...
faculty there in 1916, teaching and researching until his death in 1950, interrupted only during
World War IIWorld War II, or the Second World War , was a global military conflict which involved a majority of the world's nations, including all great powers, organized into two opposing military alliances: the Allies and the Axis...
, when he worked on the secret
Manhattan ProjectThe Manhattan Project was the codename for a project conducted during World War II to develop the first atomic bomb. The project was led by the United States, and included scientists from Denmark, The United Kingdom and Canada...
to develop the world's first
nuclear weaponA nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion...
s. From 1943 to 1946, he was chief physicist of the University of Chicago's
Metallurgical LaboratoryThe Metallurgical Laboratory or "Met Lab" at the University of Chicago was part of the World War II–era Manhattan Project, created by the United States to develop an atomic bomb...
or "Met Lab" (this lab was integrally related to the Manhattan Project: it was founded to study the materials necessary for the manufacture of atomic bombs), and in 1946 took a position as a division director at the
Argonne National LaboratoryArgonne National Laboratory is one of the United States Department of Energy's oldest and largest science and engineering research national laboratories and is the largest in size in the Midwest . The laboratory is managed by UChicago Argonne, LLC, which is composed of the University of Chicago and...
. Dempster died in 1950 in
Stuart, FloridaThis article is only about the City of Stuart in Martin County, Florida. For other nearby places, see: Martin County.Stuart is a city in Martin County, Florida, on Florida's Treasure Coast. The population was 14,633 at the 2000 census. As of 2007, the population recorded by the U.S. Census Bureau...
.
Research
In 1918, Dempster developed the first modern
mass spectrometerMass spectrometry is an analytical technique for the determination of the elemental composition of a sample or molecule. It is also used for elucidating the chemical structures of molecules, such as peptides and other chemical compounds...
, a scientific apparatus allowing physicists to identify compounds by the mass of elements in a sample, and determine the isotopic composition of elements in a sample. Dempster's mass spectrometer was over 100 times more accurate than previous versions, and established the basic theory and design of mass spectrometers that is still used to this day. Dempster's research over his career centered around the mass spectrometer and its applications, leading in 1935 to his discovery of the uranium isotope
235U. This isotope's ability to cause a rapidly expanding
fissionIn nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts, often producing free neutrons and lighter nuclei, which may eventually produce photons...
nuclear chain reactionA nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...
allowed the development of the atom bomb and
nuclear powerNuclear power is power produced from controlled nuclear reactions. Commercial plants in use to date use nuclear fission reactions....
. Dempster was also well known as an authority on positive rays.